At present, two primary methods for monitoring airborne fiber concentration exist. In the first method, airborne fibers are collected on a filter. This filter is analyzed by microscopy or chemical methods to determine the type of fibers present and to estimate airborne fiber concentration. This method suffers from the drawbacks of delayed availability of information, tediousness, inconvenience, high cost per sample, and lack of precision. Also, identification of fibers typically is performed by visual inspection, adding uncertainty to measurements for particular species of airborne fibers.
In the second method, real-time airborne fibers concentration is determined using optical techniques, in which light, attenuated by fibers passing by a light source, is analyzed. However, most of these devices do not discriminate between different species of airborne fibers and, in particular, may not provide an accurate measurement of potentially respirable fibers, particularly small glass fibers.
Because of the significant health problems posed by airborne asbestos fibers, current real-time airborne fiber monitors typically are aimed at selectively determining asbestos fiber concentration in an air sample having asbestos and other fibers. Because asbestos fibers exhibit paramagnetic properties, some existing devices preferentially align and oscillate asbestos fibers using, for example, a time-varying electric field quadruple, a hybrid electric/magnetic field, or both. The induced oscillations tend to create a characteristic scattering of an impinging light, thus identifying the oscillating fiber as asbestos. Electrostatic techniques also may be used. Examples of such devices and methods for measuring airborne particulate concentration are found in U.S. Pat. No. 3,692,412 to Chubb (1972), entitled "Apparatus for Analyzing Suspended Particles"; in U.S. Pat. No. 4,940,327, to Lillienfeld (1990), entitled "Method and Apparatus for Real-Time Asbestos Monitoring"; and in U.S. Pat. No. 5,319,575, also to Lillienfeld (1994), entitled "System and Method for Determining and Outputting Airborne Particle Concentration." Also see MIE Fiber Monitor Model FM-7400 User's Manual by MIE, Inc., Billerica, Mass.
However, because potentially harmful respirable fibers including, for example, glass fibers, often do not exhibit paramagnetism, such methods may not be appropriate. What is needed, then, is an airborne fiber concentration measuring device that can accurately determine the concentration of respirable fibers suspended in an air sample, in real time, without the need for electrostatic, magnetic or hybrid electromagnetic components.
Additionally, the Lillienfeld's device is more complicated, detects only a ;mall percentage of fibers in a given sample, and if the concentration of fibers in the sample is low or not representative of the fiber concentration in the air flow, measurement errors can result. There therefore remains a need for a fiber concentration measuring device which takes a more significant sampling of the fiber population and which is accurate at low concentration readings.